[HLSL][RootSignature] Add support for the `/Frs` compiler flag

[inbelic]

This issue tracks implementing the DXC Frs compiler option.

The behaviour should follow the specification here.

AC:

• Implement the Frs driver option in clang-dxc
• Implement Frs driver option in clang and cc1
• Create a compiler action that will output a separate DXIL Container with just the RTS0 part